Electron microscopy methods have been widely utilized in materials science owing to their excellent spatial resolution. Capable of achieving resolving powers on the order of nanometers, electron microscopy methods are crucial for profiling and characterizing materials and their nanoscale structure.1
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Cryogenic electron microscopy (cryo-EM) is a variant of electron microscopy that uses cryogenically cooled samples that have been ‘flash frozen’. Cryo-EM methods have proved revolutionary in the life sciences and structural biology, making it possible to obtain structural information on samples that could not be characterized using traditional X-Ray crystallographic methods.2
Cryo-EM has proved a similarly powerful technique in the material sciences, largely for its ability to prevent electron beam damage to very delicate samples.3 One of the major challenges when performing electron microscopy measurements, particularly for biomolecules, is avoiding structural change or damage to the sample with the intense electron beam while still maintaining sufficient electron flux to see a clear image without prolonged exposure times that also increase the risk of damage.4
In an electron microscopy measurement, a sample is scanned with a focused electron beam. Measurements can be made either in transmission, where the beam of electrons passes through a sample and is directly imaged on a detector or through the detection of secondary or scattered electrons off the axis of the electron beam.
High brightness electron beams have been essential in improving the quality of electron microscopy images but come with the drawback of increasing the rates of sample damage.
Lithium is one of the most promising materials at present for modern battery technologies is lithium. It is typically considered an example of a ‘delicate’ battery material that is somewhat challenging to measure with electron microscopy methods as it is beam-sensitive. Lithium is a highly reactive material that readily corrodes on exposure to air and has a low melting point. What this means is that during an electron microscopy experiment, the sample must be transferred to the electron microscope under vacuum conditions.
Being able to cryogenically freeze lithium serves two purposes. The first is to reduce the beam damage and provide a way of designing a transfer procedure to insert the lithium into the microscope without the degradation of the sample.5 The second is to allow for dynamical studies of what happens during battery charge and discharge to the lithium.
Cryo-EM can be used to flash freeze a sample into a particular state. If the timing of the freezing process is controlled, particular ‘snapshots’ along a chemical process or reaction can be captured, such as during the dendrite growth processes that cause structural changes during battery operation. Building up enough snapshots of different dynamical events in a battery cycle can give a complete understanding of how the battery works and operates.
The researchers who developed the cryo-EM transfer process could use this to capture the first electron microscopy images of lithium metal atoms and their interface with the solid electrolyte interphase and understand the growth direction of dendrites in the electrolyte during battery operation.5
Commercialization of lithium-based energy storage technologies means addressing issues such as dendrite growth on the lithium anode, issues with the expansion of the Si anode, oxygen crossover in Li-air cells causing unwanted degradation and shuttle effectiveness in Li-S cells.3
All of the aforementioned processes involve changes in the battery structure on the nanoscale, and so cryo-EM is an ideal technique to provide insight into the mechanisms by which these processes occur and what factors influence the stability of the lithium components to design longer-lived, more effective batteries.
As well as providing insights into how dendrite formation occurs in batteries, cryo-EM has also been useful in supporting measurements to quantify the amount of ‘dead’ lithium in batteries and identifying where the Li0 species appear with the Li+.6 One of the major issues with lithium-ion batteries is, despite their huge theoretical energy storage capabilities, this capacity is rarely realized in functioning devices.
One of the major issues hampering lithium-ion battery uptake and lifetime is the formation of Li0 or ‘dead’ lithium species that hamper the Coulombic efficiency of the material. Cryo-FIB-SEM and cryo-TEM images were able to show that the inactive Li0 had essentially become insulated and isolated from the current collectors. As soon as the structural connection became lost, the Coulombic efficiency of the battery started to decrease.
Realizing the importance of the physical and structural connection between the current collectors has led to proposals that future battery designs should deliberately engineer the lithium structure and electrolyte with columnar microstructures to prevent this occurrence and minimize the formation of the unreactive Li0 residue.
With a growing number of commercial options available for cryo-EM, from companies such as ThermoFisher and the availability of cryo-EM servies, cryo-EM is becoming an increasingly accessible technique to the material science and battery development communities. Understanding battery function at the nanoscale relies on the availability of such tools and opens the opportunity of targeted and intelligent design of future devices.
References and Further Reading
Smith, D. J. (2008). Ultimate resolution in the electron microscope? Materials Today, 11, 30–38. https://doi.org/10.1016/S1369-7021(09)70005-7
Callaway, E. (2020). The protein-imaging technique taking over structural biology. Nature, 578, 201. https://media.nature.com/original/magazine-assets/d41586-020-00341-9/d41586-020-00341-9.pdf
Ju, Z., Yuan, H., Sheng, O., Liu, T., Nai, J., Wang, Y., Liu, Y., & Tao, X. (2021). Cro-Electron Microscopy for Unveiling the Sensitive Battery Materials. Small Sci., 1, 210055.
Baker, L. A., & Rubinstein, J. L. (2010). Radiation damage in electron cryomicroscopy. In Methods in Enzymology (Vol. 481, Issue C). Elsevier Masson SAS. https://doi.org/10.1016/S0076-6879(10)81015-8
Li, Y., Li, Y., Pei, A., Yan, K., Sun, Y., Wu, C. L., Joubert, L. M., Chin, R., Koh, A. L., Yu, Y., Perrino, J., Butz, B., Chu, S., & Cui, Y. (2017). Atomic structure of sensitive battery materials and interfaces revealed by cryo–electron microscopy. Science, 358(6362), 506–510. https://doi.org/10.1126/science.aam6014
Fang, C., Li, J., Zhang, M., Zhang, Y., Yang, F., Lee, J. Z., Lee, M. H., Alvarado, J., Schroeder, M. A., Yang, Y., Lu, B., Williams, N., Ceja, M., Yang, L., Cai, M., Gu, J., Xu, K., Wang, X., & Meng, Y. S. (2019). Quantifying inactive lithium in lithium metal batteries. Nature, 572(7770), 511–515. https://doi.org/10.1038/s41586-019-1481-z